Compactible fused and atomized metal powder

ABSTRACT

METAL POWDERS PRODUCED BY FUSING AND ATOMIZING METALS SELECTED FROM GROUPS 4, 5 AND 6, THE RARE EARTHS, AND THE ACTINIUM SERIES OF THE PERIODIC TABLE AND THEIR ALLOYS ARE MADE HIGHLY COMPACTIBLE BY REDUCING THEIR RELATIVELY HIGH ENERGY STATE IN THE AS-PRODUCED CONDITION TO A LOWER ENERGY STATE ACCOMPAINED BY THE DISCHARGE OF ENERGY IN THE FORM OF HEAT. THIS REACTION IN ENERGY STATE IS ACCOMPLISHED BY CONTROLLED ANNEALING OR BY A PLURALITY OF COMPACTING AND RECRUSHING STEPS, OR A COMBINATION THEREOF.

United States .Patent Ofice 3,690,963 COMPACTIBLE FUSED AND ATOMIZEDMETAL POWDER Donald R. Spink, Waterloo, Ontario, Canada, and Allen C.Goodrich, East Aurora, N.Y., assignors t Amax Specialty Metals, Inc.,Akron, N.Y.

No Drawing. Original application Feb. 18, 1966, Ser. No. 528,390, nowPatent No. 3,498,782, dated Mar. 3, 1970. Divided and this applicationFeb. 24, 1970, Ser.

Int. Cl. 1322f 9/00 US. Cl. 148-126 3 Claims ABSTRACT OF THE DISCLOSUREThis application is a division of application Ser. No. 528,390, filedFeb. 18, 1966, nOW Pat. N0. 3,498,782.

BACKGROUND OF THE INVENTION This invention relates to compactible metalpowders and a process for making powders compactible. More particularlythis invention relates to plasma-arc produced powders of high meltingmetals such as those selected from Groups 4, 5 and 6, the rare earthsand the actinium series of the periodic table and a process for makingthese metal powders more compactible.

The necessity for metal parts capable of operating under severeconditions, such as in nuclear reactors where such parts must be highlycorrosion resistant and must possess high temperature resistance andmechanical strength, has increased the demand for parts formed fromalloys of such high melting metals as zirconium, titanium, tungsten,tantalum, uranium, and niobium. Fabrication of these metals and theiralloys has been restricted due to ease of contamination of these metalsby elements such as oxygen, nitrogen, and iron, and the difiiculty inworking the metals by conventional metal working techniques. Forexample, parts formed from zirconium based alloys frequently exhibitundesirable anisotropic properties as Well as low corrosion resistancewhen formed by conventional metal Working techniques. The economicaloptimization of corrosion properties, physical properties andfabrication of the aforementioned metals and their alloys have beenshown to lie in the use of the powder metallurgical techniques, andthese are an object of this invention.

Alloys of the aforementioned group of metals, particularly zirconiumalloys, are frequently used in the alpha stabilized form. Alphastabilized alloys are those in which the alpha phase is stabilized, byalloy additions, to temperatures above that which it normally survives.In the alpha stabilized form the eifect of conventional metal workingand heat treating operations on the mechanical and chemical propertiesof the alloy is readily apparent. An alpha stabilized alloy comprising1.2 to 1.7 weight percent tin, 0.07 to .20 weight percent iron, 0.05 to0.15 weight percent chromium, and 0.03 to 0.08 weight percent nickel,and referred to in the industry as zircaloy 2, is particularly suitablein regard to neutron absorption, corrosion resistance and mechanicalstrength. However,

3,690,963 Patented Sept. 12, 1972 when prepared by conventional metalworking techniques this alloy may exhibit mechanical weakness and poorcorrosion resistance which could cause premature failure in stressedapplications. Using conventional techniques zircaloy is frequentlyforged starting at 1700 to 1800 F. for 12 to 16" dia. ingots. Working iscompleted well above the 1450 P. so that a condition of slow coolingfrom the alpha-beta phase is present during the production of forgedbars.

The mechanical weakness may be associated with a relatively high degreeof anisotropy of properties (i.e. the finished articles have differentchemical and physical properties in one direction than in otherdirection). This slow cooling tends to form a microstructure of largealpha zirconium grains of relatively low alloy content surrounded by anetwork of coarse grains of intermetallic compounds and thismicrostructure can exhibit anisotropic properties, especially in objectshaving thin cross sections. Moreover such a microstructure has beenobserved to yield poor resistance to corrosion apparently due to theintermetallic compounds. For example, the precipitation of iron andchromium intermetallic compounds from slowly cooled alloys was found tobe the cause of intergranular corrosion in the heat affected zone ofwelds made in commercial zirconium of high impurity content.

Heat treatment is required to affect a resolution of the oifendingintermetallic compounds in order to avoid the undesirable side effectswhich may occur when such metals as zirconium, hafnium, titanium and thelike are worked by conventional techniques. It appears to be necessaryto change at least a major portion, if not all of the structure, to thebeta phase since the beta phase has a greater solubility for thealloying elements. Rapid quenching from the beta phase temperatureregion is performed to prevent the segregation of intermetallics whichmay be subsequently worked into stringers which extend through thegrains for considerable distances (as much as inch or more) causinganisotropic properties and areas of reduced corrosion resistance.

-An example of such heat treatment is disclosed in US. Pat. 2,894,866 toPicklesimer wherein a procedure is set forth for preventing theformation of intermetallic stringers and anisotropic properties duringthe fabrica- 7 tion of alpha stabilized zirconia based alloys. Thisprocess includes performing major size reduction at a malleabilizingtemperature excluding the alpha plus beta range, heating the workpieceto a temperature above 970 C. (beta range) for at least approximately 30minutes, cooling the workpiece rapidly, working at a temperature belowapproximately 500 C. to reduce the cross sectional area of the workpieceby at least 20 percent, annealing at a temperature from approximately700 C. to 810 C. for at least approximately 15 minutes and finallycooling the article thus fabricated. This involved procedure involvesmany possibilities for the introduction of undesirable interstitialelements and other contamination. Moreover this procedure does notreadily lend itself to the production of intricate final shapes normallyrequired for nuclear reactor or other applications.

Powder metallurgy offers an attractive alternate approach to theintricate shaped hardware of many industries. In the fabrication ofzircaloy 2 and other metals, by powder metallurgy techniques, theconstituents may be blended to the desired homogeneity, pressed into anintermediate high density green compact which requires a minimum of hotor cold working and annealing to provide the desired finished shape. Thepowder metallurgical technique provides a simpler method which mayrequire less costly fabricating equipment. The application of thispowder metallurgical technique and using fused and atomized particleswhich have been rapidly quenched from the beta phase excludes theformation of intermetallic stringers and coarse preferentially orientedcrystals by introducing only fine crystals which are randomly oriented.A final shape more resistant to corrosion of high purity having improvedmechanical properties and a simplified fabricating technique areadvantages which result from the availability of the compactiblemicrospherical powder which is a subject of this invention. Theapplication of plasma produced microspherical powders which had adesirable shape and chemical purity was, prior to this invention,impractical because of the poor compactibility (low compact greenstrength and low compact density) of this material.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto provide metal powder produced from high melting metals such as thoseof Groups 4, 5, and 6, the rare earths, and actinium series of theperiodic table and their alloys, which is readily and easily compactedinto a dense homogeneous, isotropic shape.

Another object is to provide a fused and atomized metal powder which canbe formed into a compact which has high green strength.

A further object is to provide a process for improving compaction ofmetal powders at moderate pressures.

-A still further object of this invention is to provide a process formaking compactible metal powder from fused and atomized metal powders.

Various other objects and advantages will appear from the followingdescription of the embodiments of this invention, and the novel featureswill be particularly pointed out in connection with the appended claims.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS We have found that metalpowders formed by fusing and atomizing metals selected from Groups 4,and 6, the rare earths, and the actinium series of the periodic tablecan be treated in accordance with this invention to form highlycompactible metal powders which are capable of being formed by powdermetallurgical techniques into articles possessing excellent chemical andmechanical properties.

The metal powders treated in accordance with this invention are producedfrom Groups 4, 5 and 6, the rare earths, and the actinium series of theperiodic table and include such metals as zirconium, hafnium, titanium,tungsten, tantalum, niobium, lanthanum, thorium, uranium, and cerium.The metal powders are produced by fusion and atomization, preferably byplasma jet techniques. In producing metal powder by plasma jettechniques, the metal or alloy to be treated may be introduced as ashape, such as a rod or bar, or as a powder, into a stream of inert gasthat has been heated to such a temperature that it is dissociated orionized. The ionized gas sweeps over the metal shape melting the surfacemetal and carrying olf the molten metal as droplets in the gas stream.The metal droplets are subsequently atomized and extremely rapidlyquenched and the resultant product is in the form of uniformly shapedmicrospheres which may range in size from about +60 mesh to about -325mesh.

There are several advantages to using metal powder produced by plasmatechniques. First, metal alloys, as well as pure metals, may beconverted into powder efficiently and economically. Other methods ofproducing alloy powders are rather cumbersome, and usually require longdiffusion treatments to insure complete alloying. Secondly, the metalpowder is of high purity and uniform shape and structure due to themethod of forming the metal droplets and their rapid quenching. Theresulting particles, known as microspheres, are less susceptible tosurface take-up of impurities because the uniform spherical shape of themicrospheres provides low surface to mass ratio. The spherical shape ofthe particles and the method of forming the microspheres aboids crystalorientation that could contribute to anisotropy. Prior to this inventionthe use of fused and atomized metal powders of metals from Groups 4, 5,and 6, the rare earths, and the actinium series of the periodic tablehas been restricted since such powders are only moderately compactibleand compacts made from these metal powders are extremely weak and mayrequire pre-sintering before they are capable of being handled. On theother hand compacts made from fused and atomized metal powders whichhave been processed according to this invention are highly compactibleand the compacts, before sintering, may achieve more than 97 percent ofthe theoretical density of the component metal.

The relatively uncompactible nature of the fused and atomized powders ofthe metals of Groups 4, 5 and 6, the rare earths, and the actiniumseries of the periodic table is attributed to the high energy state inwhich the powder particles exist in the as-produced condition. This highenergy state is presumably due to the extremely rapid quenching of themolten droplets of metal that occurs after fusing and atomizing themetal during the process for producing fused and atomized metal powder.We have found that lowering the energy state of the particles byannealing or cold working the powder in accordance with this inventionthe particles become highly compactible. This changing of the particlesfrom the high energy state to a low energy state is marked by a releaseof the energy in the form of heat and is accomplished withoutdeleteriously affecting the chemical and physical properties of themetal or alloy powder so treated.

In the practice of one embodiment of this invention, fused and atomizedmetal powders are annealed under an inert condition at a temperatureabove 300 C. but below the temperature at which the crystals change to aless corrosion resistant state. Inert conditions, as used throughoutthis specification, mean conditions wherein the environment surroundingthe particular metal or alloy being annealed is substantially unreactivewith that metal or alloy. Such an inert condition exists for examplewhen annealing in a vacuum or in an atmosphere of argon. How ever, thisterm includes annealing a metal or alloy in any atmosphere which isunreactive with the material being treated and may include annealing inair in some cases. The temperature varies with the metal or alloy beingannealed. For example when using zirconium and zirconium alloys thetemperature at which the crystals change to a state of poor corrosionresistance is approximately 870 C. Therefore when annealing zirconium orits alloys, such as zircaloy 2, according to this invention, theannealing temperature is maintained between 300 and 870 C. The upperannealing temperatures are determined from experience and by a study ofthe phase diagrams of the particular metal or alloy being treated.

The exothermic nature of the change that is coincident with the changeto greater compactibility (higher green strength and density ofcompacts) has led to the following theory. The rapidly quenched, fusedand atomized particles are in a high energy state. On heating andholding at temperatures above 300 C. (more particularly at a temperatureof approximately 780 C. for zirconium a1- loys such as zircaloy 2) anexothermic reaction accompanies the change to a lower energy state whichis more amenable to compacting. This lower energy state can also beachieved by cold working.

Thus, although it is the preferred embodiment of this invention toanneal the microspheres in order to affect the change of crystallineenergy state from the uncompactible high energy state to the compactiblelow energy state, it is within the scope of this invention to afl e'ctthe change by a multiplicity of compacting and crushing steps. It isalso within the scope of this invention to lower the particle energylevel by a combination of annealing and compacting and crushing steps.

The compacts are usually formed at a pressure of between 50,000 p.s.i.and 200,000 p.s.i. using conventional powder metallurgical techniquesand equipment.

Compactibility was determined by measuring the volume and weight of agreen compact using ASTM procedure (D70-52). Compact green strength wasdetermined by the following procedure. Five gram samples of the metalpowder being tested were pressed in a cylindrical mold having a diameterof /2 inch. These cylinders (not less than 5 per test) were dropped froma height of 36 inches into a 2000 ml. stainless steel beaker having afirmly supported fiat bottom. The contents of the beaker were carefullypoured onto an 8 mesh screen. The pieces retained on the 8 mesh screenwere weighed. This weight when divided by the total sample weight andmultiplied by 100, resulted in the compact green strength for thearticle being tested.

The object and advantages of this invention are shown in greater detailin the following examples and by reference to the appended claims.

The following example illustrates the increase in powder compactibilityobtained when fused and atomized metal powders are treated according tothis invention.

EXAMPLE 1 A fused and atomized metal powder, formed by conventionalplasma jet technique, such as described above, was prepared from azirconium alloy comprising, in addition to zirconium, l.2-1.7 weightpercent tin, .05-.l5 weight percent chromium, .07.20 weight percentiron, and .03.08 weight percent nickel.

The zirconium alloy metal powder had a mesh size ranging from +60 to 325US. Standard Mesh.

Five gram samples of the fused and atomized zirconium alloy powder werepressed at various pressures to test the compactibility of the powder inthe as-produced condition. Tests were conducted on the as-producedpowder both with and without a die lubricant. The die lubricant was apowdered fat manufactured by the Capital City Products Company,Columbus, Ohio and sold under the mark Sterotex. The results are shownin Table I.

TABLE I Green strength of compacts formed from as-produced zirconiumalloy [used and atomized powder Percent compacted Compacting pressure,No die Die The results of Table I show that even at pressures of 200,000p.s.i.g., using die lubricant, the compactibility of the as-prod1ucedpowder is relatively poor.

To improve compactibility, samples which had been pressed at 150,000p.s.i.,g. without die lubricant were recrushed and recompacted anadditional four times. The compacts were tested for green strength aftereach recompaction. Recrushing may be carried out by any conventionalmeans and although the compact may be reduced to a coarser powder thanthe original metal powder, it is preferred that crushing be continueduntil the compact is reduced to powder of substantially the same meshsize as the original powder. Recompacting was carried out at 150,000p.s.ig. Table I I shows the improvement in powder compacti'bility thatwas obtained by mechanically working the fused and atomized metal powderof Example 1.

6 TABLE II Effect of crushing and recompacting on fused and atomizedmetal powder Number of recrushing and recompacting cycles:

Green strength (percent compacted) EXAMPLE 2 To illustrate the effect ofannealing on the compactibility of the microspherical powder, samples ofzirconium microspherical powder, as produced in Example 1, were annealedin the following manner. The rnicrospherical powder was placed in a coolannealing furnace l00 C.) and the furnace was purged with argon toexclude oxygen. The temperature of the furnace was gradually increasedto 775 C.:25 C. The temperature was increased gradually because atapproximately 600 C. the powder in the furnace began to give oif energyin the form of heat and consequently less heat input was needed in orderto raise the temperature of the powder to the desired annealingtemperature. It was observed that if the furnace temperature increasewas too rapid the exothermic nature of the fused land atomized powdercaused the furnace temperature to rise above 810 C. thereby formingundesirable corrosion susceptible crystals in the microspheres and themechanical properties of articles containing this material may be verypoor.

The furnace was held at 775 0.125" C. for sufiicient time to allowsubstantially all of the energy to be discharged in the form of heat(approximately three hours) and then cooled slowly to room temperature.The annealing time may vary depending on the size of the charge and thetemperature at which the energy discharge is carried out. For examplethe energy discharge which marks the energy level change of theparticles will occur more rapidly at 775 C. than at 300 C. Five gramsamples of the annealed powder were pressed in a A2 inch diametercylindrical mold at pressures of 50,000 p.s.i. and 150,000 p.s.i.Compact green strength was measured in the same manner as in Example 1.The results are set forth in Table III.

TABLE III Compacting pressure, p.s.i.:

Percent compacted Although the examples of this specification aredirected toward fused and atomized powders of zirconium and its alloysit is within the scope of this invention to include the high meltingmetals of Groups 4, 5, and 6, the rare earths, the actinium series andtheir alloys, which are only moderately compactible when formed into apowder by fusion and atomization.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification, and this application is intended to cover any variations,uses, or adaptations of the invention. It will, therefore, be recognizedr that the invention is not to be considered as limited to the preciseembodiments shown and described but is to be interpreted as broadly aspermitted by the appended claims.

We claim:

1. A method for producing highly compactible metal powder from fused andatomized particles of metals selected from a group consisting of Groups4, 5, 6, the rare earths, and the actinium series of the periodic tableand their alloys which comprises reducing said metal particles from ahigh energy state to a lower energy state as evidenced by the release ofenergy in the form of heat whereby the particles are readily compactibleinto compacts having high green strength, wherein said fused andatomized metal powder is gradually heated under inert conditions to atemperature of at least 300 C., said powder being held at thistemperature for sufficient time to allow for the discharge of energy inthe form of heat from said powder and slowly cooling said powder to roomtemperature.

2. A method for producing highly compactible metal powder from fused andatomized particles of metals selected from a group consisting of Groups4, 5, 6, the rare earths, and the actinium series of the periodic tableand their alloys which comprises reducing said metal particles from ahigh energy state to a lower energy state as evidenced by the release ofenergy in the form of heat whereby the particles are readily compactibleinto compacts having high green strength, which comprises the steps ofgradually heating under inert conditions fused and atomized particlescomprising zirconium to a temperature of between 600 C. and 870 C.,maintaining said particles under said inert conditions at a temperaturebelow 870 C. for sufficient period of time to allow for substantially acomplete discharge of energy in form of heat from said particles, andslowly cooling said particles to room temperature whereby said particlesare reduced from a high energy state to a lower energy state therebymaking said particles compactible.

3. The method of claim 2 wherein said particles are maintained at atemperature of about 775 C. for a period of about three hours.

References Cited UNITED STATES PATENTS 2,905,547 9/1959 Yoblin l48l263,049,421 8/1962 Allen et a1. 75O.5 B 3,165,396 1/1965 Goon 75O.5 B

L. DEWAYNE RUTLEDGE, Primary Examiner.

W. W. STALLARD, Assistant Examiner US. Cl. X.R.

75O.5 B, 0.5 BB; 264-12

